Device and method for checking whether contactor provided in ESS is welded

ABSTRACT

The present invention relates a device and method for checking the welding of a contactor of an energy storage system (ESS) without blocking the flow of current including a contactor of two relays connected in parallel that is used without blocking the flow of current, a battery system controller (BSC) determining whether to use the ESS, a plurality of battery racks connected with each other and an ESS power cut-off unit for blocking the use of the ESS by receiving a command from the BSC, the contactor has a first end connected to the plurality of battery racks and a second end connected to a positive output terminal of the ESS.

TECHNICAL FIELD

The present invention relates to a device and method for checking awelding of a contactor in an energy storage system (ESS).

More particularly, the present invention relates to a device and methodfor checking a welding of a contactor without blocking the flow ofcurrent.

BACKGROUND ART

weldingRecently, due to the depletion of fossil energy and theenvironmental pollution caused by the use of fossil energy, there is agrowing interest in electrical products that can be driven usingsecondary batteries. Accordingly, as technology developments and demandson a mobile device, an electric vehicle (EV), a hybrid vehicle (HV), anenergy storage system (ESS), and an uninterruptible power supplies(UPS), and the like increase, the demand for secondary batteries as anenergy source is rapidly increasing.

In addition to the primary benefit of dramatically reducing the use offossil energy, there is no by-product of the use of energy. Therefore,these secondary batteries are attracting attention as a new energysource for eco-friendliness and energy efficiency.

Meanwhile, the plurality of secondary batteries may be connected to forma module, and the modules may be connected to form one ESS.

In the ESS formed through such a manner, a contactor is used toelectrically connect the external device with the ESS.

The contactor is artificially opened if there is a problem with the useof the ESS. Therefore, a battery system controller (BSC) of the ESSshould periodically check whether the contactor provided in the ESS iswelded.

In the related art of checking whether the contactor is welded, the flowof current in the contactor is blocked, and then a welding state of thecontactor is checked.

However, since the ESS maintains a state of being always connected tothe power grid, it is difficult to block the flow of current to checkwhether the contactor is welded.

Therefore, the present invention proposes a device and method forchecking the welding of the contactor of the ESS without blocking theflow of current.

(Patent Document) KR10-2013-0051102 A

DISCLOSURE OF THE INVENTION Technical Problem

The present invention provides a device and method for checking thewelding of a contactor of an energy storage system without blocking theflow of current.

Technical Solution

In accordance with an exemplary embodiment, an energy storage system(ESS) that is configured to be always connected to the power gridincludes a battery system controller (BSC) configured to determineWhether to use the ESS based on a battery current, voltage, andtemperature, a plurality of battery racks connected in series or inparallel with each other, a positive output terminal configured tooutput a power supplied from the plurality of battery racks to outside,and an ESS power cut-off unit configured to block the use of the ESS byreceiving a command of the BSC;

The ESS power cut-off unit may include a contactor having a first endconnected to each battery rack of the ESS and a second end connected tothe positive output terminal of the ESS, the contactor is configured toform an electrical path between the battery rack and the positive outputterminal of the ESS, and a current measuring unit positioned on anelectrical path between each battery rack and the contactor, the currentmeasuring unit is configured to measure a current of the path andtransmit the measured current to the BSC.

The contactor may include a first relay having a first end connected toan output of the current measuring unit and a second end connected thepositive output terminal of the ESS to form a first current path, asecond relay having a first end connected to the output of the currentmeasuring unit and a second end connected to the positive outputterminal of the ESS to form a second current path, a first detectionunit configured to detect whether the first relay is welded and transmita detection result to the BSC, and a second detection unit configured todetect whether the second relay is welded and transmit a detectionresult to the BSC, and the first relay and the second relay may beconnected in parallel.

The first detection unit may be a current meter having a first endconnected to the output terminal of the current measuring unit and asecond end connected to the positive output terminal of the ESS to forma parallel circuit with the first relay, the first detection unit isconfigured to measure a current when the first relay opened, and thesecond detection unit may be a current meter having a first endconnected to the output terminal of the current measuring unit and asecond end connected to the positive output terminal of the ESS to forma parallel circuit with the second relay, the second detection unit isconfigured to measure a current when the second relay is opened.

The first detection unit includes a first photo coupler, the first photocoupler is connected in parallel with the first relay and when the firstrelay is open, current flows to the first photo coupler to emit light.The first photo coupler includes a first light emitting part and a firstlight receiving part insulated from the first light emitting part, thefirst light emitting part is configured to emit light when there is nowelding in the first relay and not emit light when there is a welding,and the first light receiving part is configured to transmit anelectrical signal to the BSC when an optical signal is inputted from thefirst light emitting part.

The second detection unit includes a second photo coupler, the secondphoto coupler is connected in parallel with the second relay and whenthe second relay is open, current flows to the second photo coupler toemit light.

The second photo coupler includes a second light emitting part and asecond light receiving part insulated from the second light emittingpart, the second light emitting part is configured to emit light whenthere is no welding in the second relay and does not emit light whenthere is a welding in the second relay, the second light receiving partis configured to transmit an electrical signal to the BSC when anoptical signal is inputted from the second light emitting part.

In accordance with another exemplary embodiment, a method for detectingwhether a contactor of an ESS connected always to a power grid is weldedincludes a current measuring step of measuring an output current of thecurrent ESS by a current measuring unit, a current checking step ofchecking whether the measured output current of the ESS is less than anallowable current of a first relay and a second relay constituting thecontactor, a first relay welding checking step of opening the firstrelay and checking whether the first relay is welded using a firstdetection unit when the output current of the ESS is less than theallowable current of the first relay and the second relay, and a secondrelay welding checking step of checking whether the second relay iswelded using a second detection unit after closing the open first relayand opening the second relay.

The first relay has a first end connected to an output of the currentmeasuring unit and a second end connected to a positive output terminalof the ESS to form a first current path. The second relay has a firstend connected to the output of the current measuring unit and a secondend connected to the positive output terminal of the ESS to form asecond current path. In the first relay welding checking step, when thefirst relay is opened and the first current path is cut off and thecurrent flows only through the second current path, the first detectionunit is used to check whether the first relay is welded. In the secondrelay welding checking step, the first relay is closed and the secondrelay is open to cut off the second current path, and whether the secondrelay is welded is checked using a second detection unit while currentflows only in the first current path.

The second relay welding checking step is performed only when welding ofthe first relay is not detected and when the welding of the first relayis detected in the first relay welding checking step, a first relaywelding detection signal is transmitted to a BSC, and the second relaywelding checking step is not performed.

Advantageous Effects

The present invention can check whether the contactor of the ESS iswelded without blocking the flow of current.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart illustrating a method for checking a welding of anESS contactor according to embodiment of present invention.

FIG. 2 is a diagram illustrating an ESS according to another embodimentof the present invention.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings so that those skilledin the art may easily carry out the present invention. The presentinvention may, however, be embodied in different forms and should not beconstructed as limited to the embodiments set forth herein. Parts notrelating to description are omitted in the drawings in order to clearlydescribe the present invention and like reference numerals refer to likeelements throughout.

Although the terms “initial,” “second,” etc., may be used herein todescribe various elements, these elements should not be limited by theseterms. The above terms are used only to distinguish one component fromanother. For example, an initial component may be referred to as asecond component and vice versa without departing from the scope of thepresent invention. Terms used in this specification are used to describespecific embodiments, and are not intended to limit the scope of thepresent invention. The singular expressions include plural expressionsunless the context clearly dictates otherwise.

Throughout the specification, when a portion is referred to as being“connected” to another portion, it includes not only “directlyconnected” but also “electrically connected” with another elementtherebetween. Furthermore, when it is described that one comprises (orincludes or has) some elements, it should be understood that it maycomprise (or include or has) only those elements, or it may comprise (orinclude or have) other elements as well as those elements if there is nospecific limitation. The term “˜ing operation” or “operation of ˜ing”used throughout the specification does not mean “operation for ˜ing”.

Terms used in this specification may be currently widely used generalterms in consideration of functions in the present invention but mayvary according to the intents of those skilled in the art, precedents,or the advent of new technology. Additionally, in certain cases, theremay be terms the applicant selects arbitrarily and in this case, theirmeanings are described in a corresponding description part of thepresent invention. Accordingly, terms used in the present inventionshould be defined based on the meaning of the term and the entirecontents of the present invention instead of the simple term name.

1. Method for checking welding of ESS contactor according to embodimentof present invention.

FIG. 1 is a flowchart illustrating a method for checking a welding of anESS contactor according to embodiment of present invention.

Hereinafter, a method for checking whether the ESS contactor is weldedaccording to an embodiment of the present invention will be describedwith reference to FIG. 1.

A method for checking whether an ESS contactor is welded may include acurrent measuring step S100 of measuring an output current of thecurrent ESS by a current measuring unit, a current checking step S200 ofchecking whether the measured output current of the ESS is less than anallowable current of a first relay and a second relay constituting thecontactor, a first relay welding checking step S300 of opening a firstrelay constituting the contactor and checking whether the first relay iswelded using a first detection unit when the output current of the ESSis less than an allowable current of the first relay and the secondrelay, and a second relay welding checking step S400 of checking whetherthe second relay is welded using a second detection unit after closingthe open first relay and opening the second relay.

On the other hand, in relation to the method of checking the welding ofthe ESS contactor of the present invention, when detecting the weldingamong the two relays that constitute the contactor, wince one relay thatmeasures the welding is disconnected and only current flows through theother relay, it should check that the current value is within theallowable current range of one relay.

In other words, both relays are normally closed, so that current flowsthrough both a first current path through which current flows throughthe first relay and a second current path through which current flowsthrough the second relay.

However, in relation to the method of checking whether the contactor iswelded, since the ESS is always connected to the power supply, it isdifficult to completely block the flow of current. Therefore, in thewelding checking step S300 of the first relay to check whether the firstrelay is welded, the first relay is opened (S310), and the second relaymaintains the closed state so that the current flows only through thesecond current path and checks whether the first relay is welded (S320).

In such a way, when the current flowing in both the first and secondcurrent paths flows in the second current path as it is in the weldingchecking step S300 of the first relay, since damage to the second relaycan occur, it is necessary to check whether the current measured by thecurrent measuring unit is within the allowable range of one second relayin the current checking step S200.

In other words, based on a result of the checking in the currentchecking step S200, it is possible to check whether the contactor iswelded only when the current is in a range that can be handled by onerelay.

Meanwhile, in the first relay welding checking step S300, whether thefirst relay is welded may be checked using the first detection unit(S320). For example, the first detection unit may include a first photocoupler. Specifically, the first photo coupler may have a first lightemitting part and a first light receiving part at the same time, and thefirst light emitting part and the first light receiving part may beelectrically insulated from each other, and a signal may be transmittedby an optical signal. In relation to the principle of operation, whenthe first light emitting part of the first photo coupler emits light andthe light is incident on the first light receiving part, it becomes aconduction state. The photo coupler is unidirectional.

Therefore, since the first relay is opened (S310) in the first relaywelding checking step S300 of the present invention, it is normal thatthe light emitting part emits light as the current flows to the firstphoto coupler.

On the contrary, despite the command to open the first relay, if awelding occurs in the first relay and thus the first relay is not openedand remains closed continuously, since most of the current flows throughthe first relay path instead of the first photo coupler, no currentflows through the first photo coupler so that the first light emittingpart does not emit light.

In other words, since the first relay welding checking step S300 opensthe first relay (S310), when the first photo coupler emits light, thefirst relay is in a normal state and the fact that the first photocoupler does not emit light means that welding occurs in the firstrelay.

On the other hand, the first relay welding checking step S300 describedabove is just an example of detecting whether the first relay is weldedusing a photo coupler. The present invention is not limited to this, andvarious methods for checking whether the relay is welded may be used.

Meanwhile, in the first relay welding checking step S300, if the weldingof the first relay is not detected based on a result of checking thewelding of the first relay by the first detection unit, the second relaywelding checking step S400 may be performed.

However, when the welding of the first relay is detected in the firstrelay welding check step S300, a first relay welding detection signal istransmitted from the first detection unit to the BSC (S500), and thesecond relay welding checking step S400 may not be performed.

Specifically, the detection of the welding of the first relay means thatit is difficult for the first relay to perform its normal function. Thatis, in a state in which the first relay is failed, it is not possible tocheck whether the second relay is welded.

That is, in the present invention, the second relay welding checkingstep (S400), only the first relay is closed and the second relay isopen. Since welding occurs in the first relay, the first relay does notperform a normal function. Therefore, in order to check whether thesecond relay is welded, it is not preferable to open the second currentpath and to flow current only through the abnormal first current path.

Therefore, it is desirable not to perform the second relay weldingchecking step.

Meanwhile, in the second relay welding checking step S400, whether thesecond relay is welded may be checked using the second detection unit(S430). At this time, the first relay is closed (S410) and the secondrelay is open (S420) so that current flows only through the first path.

The second detection unit may, for example, include a photo coupler.Specifically, the second photo coupler may have a second light emittingpart and a second light receiving part at the same time, and the secondlight emitting part and the second light receiving part may beelectrically insulated from each other, and a signal may be transmittedby an optical signal. In relation to the principle of operation, when asignal is inputted to the light emitting diode in the second photocoupler, the first light emitting part emits light, and the light may bereceived by the second light receiving part to generate an electricalsignal.

Therefore, since the second relay is opened (S420) in the second relaywelding checking step (S400) of the present invention, it is normal thatthe light emitting part emits light as the current flows to the photocoupler.

On the contrary, despite the command to open the second relay, if awelding occurs in the second relay and thus the second relay remainsclosed continuously, since most of the current flows through the secondrelay path instead of the photo coupler, no current flows through thephoto coupler so that the light emitting part does not emit light.

In other words, since the second relay welding checking step opens thesecond relay, when the photo coupler emits light, the second relay is ina normal state and the fact that the photo coupler does not emit lightmeans that welding occurs in the second relay.

On the other hand, the second relay welding checking step S400 describedabove is just an example of detecting whether the second relay is weldedusing a photo coupler. The present invention is not limited to this, andvarious methods for checking whether the relay is welded may be used.

On the other hand, if the welding of the second relay is not detected inthe second relay welding checking step S400, the open second relay maybe closed (S440), and the procedure of checking whether the ESScontactor is welded may be terminated.

Meanwhile, when welding of the second relay is detected in the secondrelay welding checking step, after transmitting the second relay weldingdetection signal from the second detection unit to the BSC (S500), theabove-described an ESS contact welding checking procedure may beterminated.

2. ESS According Another Embodiment of Present Invention

FIG. 2 is a diagram illustrating an ESS according to another embodimentof the present invention.

Hereinafter, an ESS according to another embodiment of the presentinvention will be described with reference to FIG. 2.

An ESS 10 according to another embodiment of the present invention isalways connected to the power grid.

Therefore, it is impossible to completely block the current flowingthrough the ESS 10.

On the other hand, the ESS 10 may include a BSC 200 for determiningwhether to use the ESS based on the battery current, voltage, andtemperature, a plurality of battery racks mutually connected in seriesor in parallel with each other, a positive output terminal foroutputting power supplied from the plurality of battery racks to theoutside, and an ESS power cut-off unit 100 for blocking the use of theESS in response to a command of the BSC.

The ESS power cut-off unit 100 may include a contactor whose one end isconnected to the battery rack of the ESS and other end is connected tothe positive output terminal of the ESS 10 and that forms an electricalpath between the battery rack and a positive output terminal of the ESSand a current measuring unit 110 disposed on an electrical path betweenthe battery rack and the contactor to measure a current of the path andtransmit the current to the BSC 200.

On the other hand, when the ESS of the present invention checks thewelding of the two relays constituting the contactor, one relay to checkfor welding is disconnected, and the current only flows through theother relay, so that it is necessary to check that the current value iswithin the allowable current range of one relay.

In other words, both relays are normally closed, so that current flowsthrough both a first current path through which current flows throughthe first relay and a second current path through which current flowsthrough the second relay.

However, when checking whether the contactor is welded, for example,when checking whether the first relay is welded, the first relay is open(S310), and the second relay is kept closed so that the current flowsonly through the second current path to check whether the first relay iswelded.

That is, since the current flows only through the second relay whenchecking the welding of the first relay, the output current of the ESSshould be within the range allowed by the second relay.

Meanwhile, the BSC 200 may periodically check whether the contactor 120is welded.

Specifically, the contactor 120 may include a first relay 121 having oneend connected to an output terminal of the current measuring unit 110and the other end connected to a positive output terminal of the ESS toform a first current path 11, a second relay 122 having one endconnected to an output terminal of the current measuring unit 110 andthe other end connected to a positive output terminal of the ESS to forma second current path 12, a first detection unit 123 that detectswhether the first relay 121 is welded and transmits a detection resultto the BSC 200, and a second detection unit 124 that detects whether thesecond relay 122 is welded and transmits a detection result to the BSC200.

Meanwhile, in FIG. 2, the reason that the connection relationshipbetween the input and output terminals of the first and second detectionunits is not clearly implemented is because the connection relationshipwith the first and second relays is determined according to thecomponents used as the first and second detection units.

For example, when the first and second detection units are configured asphoto couples, the first and second detection units may be connected inparallel with the first and second relays, respectively.

Meanwhile, when the first detection unit 123 detects whether the firstrelay 121 is welded, the first current path 11 is blocked, and currentcan flow only in the second current path 12. When the second detectionunit 124 detects whether the second relay 122 is welded, the secondcurrent path 12 is blocked, and current can flow only in the firstcurrent path 11.

By connecting the first and second relays in parallel as describedabove, it is possible to check the welding of the first and secondrelays sequentially without interrupting the flow of current.

In other words, the first detection unit has one end connected to theoutput terminal of the current measuring unit and the other endconnected to the positive output terminal of the ESS. The firstdetection unit may configure a parallel circuit with the first relay andif the first relay is open, the first detection unit may be a currentmeter that measures current on a parallel path.

For example, the first detection unit 123 may include a first photocoupler. Specifically, the first photo coupler may have a first lightemitting part and a first light receiving part at the same time, and thefirst light emitting part and the first light receiving part may beelectrically insulated from each other, and a signal may be transmittedby an optical signal. In relation to the principle of operation, when acurrent flows through the first photo coupler, the first light emittingpart may emit light and generate an electrical signal when the light isreceived by the first light receiving part. In other words, when thefirst relay is open, the first photo coupler is connected in parallelwith the first relay so that current flows to the first photo coupler toemit light.

Therefore, since the first detection unit 123 of the present inventiondetects the welding of the first relay while the first relay 121 isopened, so that as the current flows to the first photo coupler, thefirst light emitting part emits light.

Meanwhile, the first light receiving part of the first detection unit123 may receive an optical signal from the first light emitting part,convert the received optical signal into an electrical signal, andtransmit the electrical signal to the BSC 200. On the other hand, whenwelding occurs in the first relay 121, it is preferable not to detectwhether the second relay 122 is welded.

Specifically, the detection of the welding of the first relay 121 meansthat it is difficult for the first relay 121 to perform its normalfunction. That is, in the present invention, when checking whether thesecond relay is welded, only the first relay is closed and the secondrelay is open. Since welding occurs in the first relay, the first relaydoes not perform a normal function. Therefore, in order to check whetherthe second relay is welded, it is not preferable to open the secondcurrent path 12 and to flow current only through the abnormal firstcurrent path 11.

Therefore, it is preferable not to detect whether the second relay 122is welded.

Meanwhile, when welding does not occur in the first relay 121, whetherthe second relay 122 is welded may be checked.

Specifically, when checking whether the first relay 121 is welded, thefirst relay 121 may be closed to form the first current path 11, and thesecond relay 122 may be opened to cut off the second current path 12.

Meanwhile, the details of checking whether the second relay 122 iswelded in the second detection unit 124 may be the same as the procedureof detecting whether the first relay 121 is welded in the firstdetection unit 123 described above.

In detail, the second detection unit has one end connected to the outputterminal of the current measuring unit and the other end connected tothe positive output terminal of the ESS. The second detection unit mayconfigure a parallel circuit with the first relay and if the secondrelay is open, the second detection unit may be a current meter thatmeasures current on a parallel path.

For example, the second detection unit 123 may include a second photocoupler. Specifically, the second photo coupler may have a second lightemitting part and a second light receiving part at the same time, andthe second light emitting part and the second light receiving part maybe electrically insulated from each other, and a signal may betransmitted by an optical signal. In relation to the principle ofoperation, when a current flows through the second photo coupler, thesecond light emitting part may emit light and generate an electricalsignal when the light is received by the second light receiving part. Inother words, when the second relay is open, the second photo coupler isconnected in parallel with the second relay so that current flows to thesecond photo coupler to emit light.

Therefore, since the second detection unit 123 of the present inventiondetects the welding of the second relay while the second relay 122 isopened, so that as the current flows to the second photo coupler, thesecond light emitting part emits light.

Meanwhile, the second light receiving part of the second detection unit124 may receive an optical signal from the second light emitting part,convert the received optical signal into an electrical signal, andtransmit the electrical signal to the BSC 200.

On the other hand, when it is checked that there is no welding of thesecond relay 122 in the second detection unit 124, the second relay maybe closed to allow current to flow in both of the first and secondcurrent paths.

On the other hand, although the technical idea of the present inventionis specifically described with reference to the above embodiments, itshould be noted that the above embodiments are for the purpose ofexplanation and not for the purpose of limitation. It will be apparentto those skilled in the art that various modifications and variationsmay be made in the present invention without departing from the spiritand scope of the invention.

The invention claimed is:
 1. An energy storage system (ESS) configuredto be permanently connected to a power grid, the ESS comprising: abattery system controller (BSC) configured to determine whether to usethe ESS based on a battery current, voltage, and temperature; aplurality of battery racks connected in series or in parallel with eachother; a positive output terminal configured to output a power suppliedfrom the plurality of battery racks outside; a negative output terminal;and an ESS power cut-off unit configured to block the use of the ESS byreceiving a command from the BSC; wherein the ESS power cut-off unitcomprises: a contactor having a first end connected to each battery rackof the ESS and a second end connected to the positive output terminal ofthe ESS, the contactor being configured to form an electrical pathbetween each battery rack and the positive output terminal of the ESS;and a current measuring unit positioned on an electrical path betweeneach battery rack and the contactor, the current measuring unit beingconfigured to measure a current of the electrical path and transmit themeasured current to the BSC, wherein the contactor comprises: a firstrelay having a first end connected to an output of the current measuringunit and a second end directly connected to the positive output terminalof the ESS to form a first current path; a second relay having a firstend connected to the output of the current measuring sensor and a secondend directly connected to the positive output terminal of the ESS toform a second current path; a first detection unit configured to detectwhether the first relay is welded and transmit a detection result to theBSC; and a second detection unit configured to detect whether the secondrelay is welded and transmit a detection result to the BSC; wherein thenegative output terminal of the ESS is connected to a negative terminalof the plurality of battery racks, wherein the first detection unitincludes a first photo coupler, and wherein the second detection unitincludes a second photo coupler.
 2. The ESS of claim 1, wherein thefirst relay and the second relay are connected in parallel.
 3. The ESSof claim 2, wherein the first detection unit is a current meter having afirst end connected to the output terminal of the current measuring unitand a second end connected to the positive output terminal of the ESS toform a parallel circuit with the first relay, and wherein the seconddetection unit is a current meter having a first end connected to theoutput terminal of the current measuring unit and a second end connectedto the positive output terminal of the ESS to form a parallel circuitwith the second relay.
 4. The ESS of claim 3, wherein the first photocoupler is connected in parallel with the first relay and when the firstrelay is open, current flows to the first photo coupler to emit light,and wherein the second photo coupler is connected in parallel with thesecond relay and when the second relay is open, current flows to thesecond photo coupler to emit light.
 5. The ESS of claim 4, wherein thefirst photo coupler comprises a first light emitting part and a firstlight receiving part insulated from the first light emitting part,wherein the first light emitting part is configured to emit light whenthere is no welding in the first relay and not emit light when there isa welding in the first relay, and wherein the first light receiving partis configured to transmit an electrical signal to the BSC when anoptical signal is inputted from the first light emitting part.
 6. TheESS of claim 4, wherein the second photo coupler is connected inparallel with the second relay and when the second relay is open,current flows to the second photo coupler to emit light, wherein thesecond photo coupler comprises a second light emitting part and a secondlight receiving part insulated from the second light emitting part,wherein the second light emitting part is configured to emit light whenthere is no welding in the second relay and not emit light when there isa welding in the second relay, and wherein the second light receivingpart is configured to transmit an electrical signal to the BSC when anoptical signal is inputted from the second light emitting part.
 7. Amethod for detecting whether a contactor of an energy storage system(ESS) is welded, the method comprising: providing the ESS, the ESS beingpermanently connected to a power grid; a current measuring step ofmeasuring an output current of the ESS by a current measuring unit; acurrent checking step of checking whether the measured output current ofthe ESS is less than an allowable current of a first relay and a secondrelay constituting the contactor of the ESS; a first relay weldingchecking step of opening the first relay while maintaining the secondrelay in a closed state and checking whether the first relay is weldedusing a first detection unit when the output current of the ESS is lessthan the allowable current of the first relay and the second relay; anda second relay welding checking step of checking whether the secondrelay is welded using a second detection unit while maintaining thefirst relay in a closed state and opening the second relay, wherein thefirst relay has a first end connected to an output of the currentmeasuring unit and a second end directly connected to a positive outputterminal of the ESS to form a first current path, wherein the secondrelay has a first end connected to the output of the current measuringunit and a second end directly connected to the positive output terminalof the ESS to form a second current path, wherein a negative outputterminal of the ESS is connected to a negative terminal of a pluralityof battery racks, wherein in the first relay welding checking step, whenthe first relay is opened and the first current path is cut off and thecurrent flows only through the second current path, the first detectionunit is used to check whether the first relay is welded, wherein in thesecond relay welding checking step, the first relay is closed and thesecond relay is open to cut off the second current path, and whether thesecond relay is welded is checked using a second detection unit whilecurrent flows only in the first current path, wherein the firstdetection unit includes a first photo coupler, and wherein the seconddetection unit includes a second photo coupler.
 8. The method of claim7, wherein the second relay welding checking step is performed only whenthe welding of the first relay is not detected.
 9. The method of claim7, wherein when the welding of the first relay is detected in the firstrelay welding checking step, a first relay welding detection signal istransmitted to a battery system controller (BSC), and the second relaywelding checking step is not performed.